Method of producing valve metal electrode with valve metal oxide semiconductive coating having a chlorine discharge catalyst in said coating

ABSTRACT

1. THE METHOD OF PRODUCING AN ELECTRODE OF A VALVE METAL BASE FROM THE GROUP CONSISTING OF TITANIUM AND TANTALUM WHICH COMPRISES APPLYING A COATING MIXTURE IN LIQUID FORM TO SAID VALVE METAL BASE, WHICH ON HEATING FORMS AN OXIDE LAYER ON SAID BASE, 39.2% TO 78% OF WHICH COMPRISES AN OXIDE OF TITANIUM, 6.4% TO 47.45% OF WHICH COMPRISES AN OXIDE OF A PLATINUM GROUP METAL AND 1% TO 17.7% OF WHICH FORMS AN OXIDE OF A DOPING METAL FROM THE GROUP CONSISTING OF TIN, VANADIUM, LANTHANUM, COBALT, AND MIXTURES THEREOF, THE SAID PERCENTAGES BEING BASED UPON THE WEIGHT OF THE METALS IN SAID OXIDES, APPLYING SAID COATING IN SEVERAL SEPARATE LAYERS AND HEATING THE COATING ON THE VALVE METAL BASE BETWEEN THE APPLICATION OF EACH LAYER.

United States Patent METHOD OF PRODUCING VALVE METAL ELEC- TRODE WITH VALVE METAL OXIDE SEMI- CGNDUCTIVE COATING HAVING A CHLORINE DISCHARGE CATALYST IN SAID COATING Giuseppe Bianchi, Milan, ltaly, Vittorio de Nora, Nassau, Bahama llslands, and Patrizio Gallone and Antonio Nidola, Milan, Italy, assignors to Electronor Corporation, Panama City, Panama N 0 Drawing. Continuation-impart of applications Ser. No. 690,407, Dec. 14, 1967, and Ser. No. 771,665, Oct. 29, 1968. This application May 10, 1971, Ser. No. 141,946

Int. Cl. B01k 3/04 US. Cl. 204-290 F 15 Claims ABSTRACT OF THE DISCLOSURE Describes chlorine resistant metal electrodes, preferably of valve metals such as titanium and tantalum, having coatings of mixed metal oxides, preferably valve metal oxides, which have been doped to provide semi-conducting surfaces on the electrodes, which coatings also have the capacity to catalyze chlorine discharge from the electrodes and to resist corrosive conditions in a chlorine call.

This application is a continuation-in-part of our copending applications Ser. No. 690,407, filed Dec. 14, 1967 now Pat. No. 3,616,445 and Ser. No. 771,665, filed Oct. 29, 1968.

This invention relates to valve metal electrodes having a semi-conductive coating of titanium dioxide or tantalum oxide or other metal oxides, which is sufliciently conductive to avoid, for a long period of time, the passivation which takes place in valve metal electrodes or in valve metal electrodes having a coating of a platinum group metal thereon, when used in electrolysis processes such as, for example, the production of chlorine, and in which the semi-conductive titanium dioxide or tantalum oxide or other metal oxide coating also contains an electrocatalytic material sufiicient, for example, to catalyze chlo rine discharge from the electrode. The electrode is sufficiently conductive to conduct electrolysis current from the electrode base to an electrolyte at continued high amperage and lower overvoltage for chlorine discharge for long periods of time.

The electrodes of our invention may be used for the electrolysis of lithium, sodium and potassium chlorides, bromides and iodides and more generally for the electrolysis of halogenides, for the electrolysis of other salts which undergo decomposition under electrolysis conditions, for the electrolysis of H01 solutions and for the electrolysis of water, etc. They may also be used for other purposes such as other processes in which an electrolysis current is passed through an electrolyte for the purpose of decomposing the electrolyte, for carrying out organic oxidations and reductions, for cathodic protection and in other electrolysis processes. They may be used in mercury or diaphragm electrolysis cells and may take other forms than those specifically described. The electrodes of our invention are particularly useful as anodes for the electrolysis of sodium chloride brines in mercury cells and diaphragm cells as they have the ability to catalyze the oxidation of dissolved chloride ions to molecular chlorine gas and to liberate chlorine at low anode voltages essentially throughout the life of the titanium dioxide or tantalum oxide or other metal oxide semi-conductor coating and have a low Wear rate (loss of conductor coating metal per ton of chlorine produced).

Valve metals, such as titanium, tantalum, zirconium, molybdenum, niobium and tungsten, have the capacity to conduct current in the anodic direction and to resist the passage of current in the cathodic direction and are sufiiciently resistant to the electrolyte and conditions within an electrolytic cell used, for example, for the production of chlorine and caustic soda, to be used as electrodes in electrolytic processes. In the anodic direction, however, their resistance to the passage of current goes up rapidly, due to the formation of an oxide layer thereon, so that it is no longer possible to conduct current to the electrolyte in any substantial amount without substantial increases in voltage which makes continued use of uncoated valve metal anodes in the electrolytic process uneconomical.

When valve metal anodes are coated with platinum or platinum group metal conductors, these too become passivated causing a rapid rise in potential after being used for a short time at sufficiently high current density under chlorine discharge. This rise in potential is probably due to the deposition of an adsorbed layer of oxygen on the platinum group metal electrodes and indicates that the anodic oxidation of the dissolved chlorine ion to molecular chlorine gas (electrocatalytic activity) will proceed only at a higher overvoltage because of the diminished catalytic activity of the electrode surface.

Attempts to overcome this passivation (after a short period of use) by providing titanium or tantalum anodes with a coating of a platinum group metal applied by electro-deposition or by thermal processes, but essentially covering the entire face of the titanium anode facing the cathode, have not been commercially successful. The coatings did not always adhere properly and the consumption of the platinum group metal was high and the results were unsatisfactory.

It has long been known that rutile or titanium dioxide and tantalum oxide have semi-conducting properties, either when doped with traces of other elements or compounds which disturb the lattice structure and change the conductivity of the titanium dioxide or tantalum oxide, or when the lattice is disturbed by the removal of oxygen from the titanium dioxide or tantalum oxide crystal. Titanium dioxide has been doped with tantalum, niobium, chromium, vanadium, tin, nickel and iron oxides and other materials to change the electrical conducting or the semi-conducting properties of the titanium dioxide and has been rendered semi-conducting by changing the stoichiometric balance of the TiO' crystals, by removing oxygen from the crystal lattice. Likewise, Ta O films have had their conductivity altered by ultraviolet radiation and by other methods.

It is known, for example, that when chemically pure titanium dioxide is doped with 1 mole percent of Nb O its specific conductance is increased from 60 ohmcm. 10' to 330.000 ohm-cm. 10 when measured at 250 C., a 5500 fold increase. Likewise, when chemically pure titanium dioxide is doped with 1 mole percent of Ta O the specific conductance of the TiO is increased 4166 fold. No one, however, has suggested the use of doped titanium dioxide or tantalum oxide to provide a conductive or semi-conductive face on a valve metal electrode for use in electrochemical reactions, nor has anyone suggested incorporating an electrocatalytic agent in such doped semi-conducting coatings to promote chlorine discharge from the anode.

Other metal oxides than TiO and Ta O when intimately mixed and heated together have the property of forming semi-conductors, particularly mixed oxides of metals belonging to adjacent groups in the Periodic Table.

Various theories have been advanced to explain the conductive or semi-conductive properties of doped or undoped titanium dioxide, also for Ta O See, for example, Grant, Review of Modern Physics, Vol. .1, page 646 (1959'); Frederikse, Journal of Applied Physics,

Supplement to Vol. 32, No. 10, page 221 (1961) and Vermilyea, Journal of the Electrochemical Society, Vol. 104, page 212 (1957), but there appears to be no general agreement as to what gives doped titanium dioxide and tantalum oxide their properties of semi-conduction. When other mixed metal oxides are used to produce semiconductors, it seems probable that oxides of one metal belonging to an adjacent group in the Periodic Table penetrates into the crystal lattice of the other metal oxide by solid solution to act as a doping oxide which disturbs the stoichiometric structure of the crystals of one of the metal oxides to give the mixed oxide coating its semiconducting properties.

The doping oxide is usually used in amounts of less than 50 mole percent of the doped oxide and should be of a greater or lesser normal valence than the oxide to be doped. Oxides of the same valence and substantially the same atomic radii and lattice parameters as TiO or I ado not act as doping agents for TiO or Ta O' but may form mixed crystals with Ti0 or Ta O The doping oxide as well as the doped oxide must be resistant to the conditions encountered in an electrolysis cell used for any givenv purpose and must be capable of protecting any electrocatalytic material incorporated in the coating.

One of the objects of this invention is to provide an electrode having a metal base and a semi-conducting mixed metal oxide coating over part or all of said base, sufiicient to conduct an electrolysis current from said base to an electrolyte over long periods of time without passivation.

Another object is to provide an anode having a valve metal base with a coating over part or all of the surface thereof, consisting primarily of titanium dioxide or tantalum oxide which has conducting or semi-conducting properties suflicient to conduct an electrolysis current from the base to an electrolyte over long periods of time without passivation.

Another object of the invention is to provide a valve metal electrode having a conducting surface consisting primarily of titanium dioxide or doped titanium dioxide or tantalum oxide or doped tantalum oxide or mixed metal oxides from adjacent groups in the Periodic Table.

Another object of our invention is to provide a valve metal electrode having a semi-conductive coating consisting primarily of titanium dioxide or tantalum oxide or mixed metal oxides in which the semi-conductive coating has an electrocatalytic chlorine discharge catalyst incorporated therein or has the properties of catalyzing chlorine discharge from the surface of the electrode without increase in overvoltage as hereindefined over long periods of time.

Another object of our invention is to provide a metal electrode having a semi-conducting face of doped titanium dioxide or doped tantalum oxide or doped metal oxides in which the metal oxide and the doping oxide are baked on the cleaned electrode face to cause a solid solution to be formed between the titanium dioxide or tantalum oxide or the metal oxide and the doping composition which will resist separation of the semi-conducting face from the metal electrode base.

Another object of our invention is to provide a valve metal electrode having a semi-conducting face of doped titanium dioxide or doped tantalum oxide or other doped metal oxide in which the doping composition and the doped metal oxide are baked on the cleaned electrode face in multiple layers to cause a solid solution to be formed between the titanium dioxide, tantalum oxide or other metal oxide and the doping composition and any electrocatalytic agent incorporated in the coating.

Another object of our invention is to provide a valve metal electrode with a valve metal oxide semi-conductor coating which will have greater adherence to the base than the platinum group metal coatings of the prior art.

Various other objects and advantages of our invention will appear as this description procceds.

In general, we prefer to make a solution of the semiconductor metal and the doping composition in such form that when applied and baked on the cleaned valve metal electrode the solution will form Ti0 plus doping oxide or Ta O plus doping oxide or other metal oxide plus doping oxide and to bake this composition on the valve metal electrode in multiple layers so as to form a solid solution of the TiO Ta O or other metal oxide and the doping oxide on the face of the electrode which will have the desired semi-conducting properties, will have electrocatalytic properties and will continue chlorine discharge without increase in overvoltage over long periods of time. Any solutions or compounds which on baking will form Ti0 plus doping oxide, Ta O plus doping oxide or other metal oxide plus doping oxide may be used, such as, chlorides, nitrates, sulfides, etc., and the solutions given below are only by way of example.

Overvoltage as used above may be defined as the voltage in excess of the reversible or equilibrium which must be applied to cause the electrode reaction to take place at the desired rate. Chlorine overvoltage varies with the anode material and its physical condition. It increases with anode current density but decreases with increase in temperature.

Titanium dioxide, tantalum oxide and other metal oxide semi-conductor faces may be produced by doping titanium dioxide, tantalum oxide and other metal oxide crystals with various doping compositions or by disturbing the stoichiometric lattice by removing oxygen therefrom to cause the TiO Ta O or other metal oxides to become semi-conductive. Because of the tendency of the TiO Ta O or other metal oxide crystals to become reoxidized, we prefer to form the semi-conductive faces on our electrodes by the use of doping compositions which on baking form solid solutions with the TiO Ta O or other metal oxide crystals which are more resistant to change during electrolysis processes. However, semiconducting coatings produced by withdrawing oxygen from the TiO Ta O or other oxide lattices to cause lattice defects or deficiencies may be used on our electrodes.

Various doping materials which introduce impurities into the TiO and Ta O crystals to make them semi-conductive, may be used to increase the conductivity and electrocatalytic properties of the TiO and Ta O layer on the electrode, such as, W0 P 0 Sb O V 0 Ta O Nb O B 0 Cr O BeO, Na O, CaO, SrO, RuO IrO PbO2, 0302, Pt02, A1102, AgO SD02, A1203, and mixtures thereof. The doping materials for TiO for example, y be 3, 2 5 z s, 2 5 2 5, z s, 2 3 CI'2O3, BeO, Na O, C30, SPO, M003, P1303, A1102, Agog, SnO Fe O NiO, C0 0 SnO LaO and mixtures thereof (with or without RuO IrOg, OsO PtO and other platinum group metals as electrocatalytic agents). The doping materials for Ta O may be, for example W 0 BeO, Na O, C30, SrO, R IP02, Pb02, 0802, PtOz, A1102, AgO SnO PtO and mixtures thereof. The doping oxide should be of a higher or lower normal valence than TiO or Ta O although the valences themselves may vary with the condition of the compound the doping oxide is in. The presence of impurities in commercial titanium and tantalum may affect the conductivity or semi-conductivity of the oxides of these metals. In the case of TiO the oxides of the platinum group metals (i.e., platinum, ruthenium, iridium, palladium, osmium and rhodium) act mainly electrocatalytically since they have the same valence and tetragonal rutile-type structure with similar unit cell dimensions and approximately the same cationic radii (0.68 A.) as Ti0 crystals. RuO (0.65 A.) and IrO (0.66 A.) are especially suitable as electrocatalysts in this context. IrO forms solid solutions in TiO up to about 5 mole percent IrO when heated together at 1040 C. At lower temperatures, the amount of 110 which will form solid solutions in TiO is lower but the amount of platinum metal oxide group which is not in solid solution continues to act as a catalyst for chlorine discharge.

Oxides of metals from Group VIII of the Periodic Table of elements as well as oxides of metals of Group VB, Group VIB, oxides of metals from Group 13 and oxides of elements from Group VA, as well as mixtures of these oxides capable on baking of forming doped semiconductive crystals of TiO and Ta O and of interrupting the crystal lattice of TiO and Ta O may be used to form semi-conductor and electrocatalytic materials may be added to the valve metal electrode coatings.

In forming semi-conductor coatings for valve metal electrodes from other metal oxides, it is preferable to use mixed oxides of metals, or materials which form mixed oxides of metals, from adjacent groups of the Periodic Table, such as, for example, iron and rhenium; titanium, tantalum and vanadium; titanium and lanthanum. Other oxides which may be used are manganese and tin; molybdenum and iron; cobalt and antimony; rhenium and manganese and other metal oxide compositions.

The percentage of the doping compositions may vary from 0.10 to 50% of the TiO Ta O or other metal oxide and surprising increases in conductivity of the TiO Ta O or other metal oxide facing can be gotten with as little as 0.25 to 1 weight percent of the doping composition to the TiO;;, Ta O or other metal oxide in the conductor face on the electrode. In addition to the doping metal oxide, we prefer to provide a coating on our anodes which will catalyze chlorine discharge without material overvoltage, if the electrode is to be used for chlorine production.

The semi-conductive coating of our invention may be applied in various ways, and to various forms of titanium or tantalum base anodes, such as solid rolled massive perforated titanium plates, slitted, reticulated, titanium plates, titanium mesh and rolled titanium mesh, woven titanium wire or screen, titanium rods and bars or similar tantalum and other metal plates and shapes. Our preferred method of application is by chemi-deposition in the form of solutions painted, dipped or sprayed on or applied as curtain or electrostatic spray coatings, baked on the anode base, but other methods of application, including electrophoretic deposition or electrodeposition, may be used. Care must be taken that no air bubbles are entrapped in the coating and that the heating temperature is below that which causes warping of the base material.

The spectrum of doped TiO' samples shows that the foreign ion replaces the Ti ion on a regular lattice site and causes a hyperfine splitting in accordance with the nuclear spin of the substituting element.

In all applications, the titanium, tantalum or other metal base is preferably cleaned and free of oxide or other scale. This cleaning can be done in any way, by mechanical or chemical cleaning, such as, by sand blasting, etching, pickling or the like.

The following examples are by Way of illustration only and various modifications and changes may be made in the compositions and form of solutions given, and in the baking procedure used and in other steps within the scope of our invention.

EXAMPLE I An expanded titanium anode plate, with a surface of 50 cm projected area, was cleaned by boiling at reflux temperature of 110 C. in a 20% solution of hydrochloric acid for 40 minutes. It was then given a liquid coating containing the following materials:

Ruthenium as RuC1 .H O mg. (metal) Iridium as (NH IrC1 10 mg. (metal) Titanium as TiC1 56 mg. (metal) Formamide (HCONH 10 to 12 drops Hydrogen peroxide (H O 30%) 3 to 4 drops The coating was prepared by first blending or mixing the ruthenium and iridium salts containing the required amount of Ru and Ir in a 2 molar solution of hydrochloric acid (5 ml. are sufiicient for the above amounts) and allowing the mixture to dry at a temperature not higher than 50 C. until a dry precipitate is formed. Formamide is then added to the dry salt mixture at about 40 C. to dissolve the mixture. The titanium chloride, TiC1 dissolved in hydrochloric acid (15% rength commercial solution), is added to the dissolved Ru-Ir salt mixture and a few drops of hydrogen peroxide (30% H 0 are added, sufficient to make the solution turn from the blue color of the commercial solution of TiC1 to an orange coor.

This coating mixture was applied to both sides of the cleaned titanium anode base, by brush, in eight subsequent layers, taking care to brush the coating into the interstices of the expanded plate. After applying each layer, the anode was heated in an oven under forced air circulation at a temperature between 300 and 350 C. for 10 to 15 minutes, followed by fast natural cooling in air between each of the first seven layers, and after the eighth layer was applied the anode Was heated at 450 C. for one hour under forced air circulation and then cooled.

The amounts of the three metals in the coating correspond to the weight ratios of 13.15% Ir, 13.15% Ru and 73.7% Ti and the amount of noble metal in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode area. It is believed that the improved qualities of this anode are due to the fact that although the three metals in the coating mixture are originally present as chlorides, they are co-deposited on the titanium base in oxide form. Other solutions which will deposit the metals in oxide form may of course be used. In accelerated testing, the anode of this example showed a weight loss of zero after three current reversals, a loss of 0.152 mg./cm. after three amalgam dips as against a weight loss of 0.93 mg./cm. of a similar titanium base anode covered with ruthenium oxide. After 2,000 hours of operation this anode showed a weight increase of 0.7 mg./cm. whereas similar anodes covered with a layer of platinum or ruthenium oxide showed substantial weight losses. The weight increase had apparently become stabilized.

EXAMPLE II The coating mixture was applied to a cleaned titanium anode base of the same dimensions as in Example II according to the same procedure. The applied mixture consisted of the following amounts:

Ruthenium as RuCl .H O 20 mg. (metal) Iridium as (NH IrCl 48 mg. (metal) Titanium as TiCl 10 to 12 drops Formamide (HCONH 3 to 4 drops Hydrogen peroxide (H 0 30%) 20 mg. (metal) The procedure for compounding the coating and applying it to the titanium base was the same as in Example II. The quantities of the three metals in this mixture corresponded to the weight ratios of 22.6% Ir, 22.6% Ru and 54.8% Ti and the amount of noble metal oxide in the active coating corresponded to 0.4 mg. Ir, and 0.4 mg. Ru per square centimeter of the active electrode area. After 2,300 hours of operation this anode showed a weight increase of 0.9 mg./cm which had apparently become stabilized.

EXAMPLE III Before being coated, a titanium anode substrate after pro-etching as described in Example II, was immersed in a solution composed of 1 molar solution of H 0 plus a 1 molar solution of NaOH at 20 to 30 C. for two days. The surface of the titanium was thus converted to a thin layer of black titanium oxide.

The coating mixture of the same composition as given in Example II was used, except that isopropyl alcohol wa sused as the solvent in place of forrnamide. The use of isopropyl alcohol resulted in a more uniform distribution of the coating films on the black titanium oxide substrate than when formamide was used as the solvent.

The presence of iridium as Ir in the mixed TiO -RuO crystals of the coating of Examples I, II and III is beneficial for chlorine evolution because of its hindrance elfect on oxygen vacancy saturation. The conductivity of the mixed TiO RuO oxides is due to oxygen vacancies and free electrons, and when the oxygen vacancies are saturated by oxygen evolution within an electrolysis cell, the conductivity of the coating on the electrode decreases.

EXAMPLE IV An expanded titanium anode plate of the same size as in the former examples was submitted to the cleaning and etching procedure as described above and then given a liquid coating containing the following materials:

Ruthenium as RuCl .H O 10 mg. (metal) Iridium as :IrCL; 10 mg. (metal) Tantalum as TaCl 80 mg. (metal) Isopropyl alcohol 5 drops Hydrochloric acid (20%) 5 ml.

The coating was prepared by first blending or mixing the ruthenium and iridium salts in 5 ml. of 20% HCl. The volume of this solution was then reduced to about onefifth by heating at a temperature of 85 C. The required amount of TaCl was dissolved in boiling 20% HCl so as to form a solution containing about 8% TaCl by weight. The two solutions were mixed together and the overall Volume reduced to about one-half by heating at 60 C. The specified quantity of isopropyl alcohol was then added.

The coating mixture was applied to both sides of the cleaned titanium anode base in eight subsequent layers and following the same heating and cooling procedure between each coat and after the final coat as described in Example I.

The amounts of the three metals in the coating correspond to the weight ratios of 10% Ru, 10% Ir and 80% Ta, and the amount of noble metal in the coating corresponds to 0.2 mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode area. In accelerated testing, this anode showed a weight loss of 0.0207 mg./cm. after three current reversals and a loss of 0.0138 after two amalgam dips. After 514 hours of operation, this anode showed a weight decrease of 0.097 mg./cm.

EXAMPLE V Titanium trichloride in HCl solution is dissolved in methanol, the TiCl is converted to the pertitanate by the addition of H 0 This conversion is indicated by a change in color from TiCl (purple) to T i 0 (orange). An excess of H 0 is used to insure complete conversion to the pertitanate. Sufiicient RuCl .3H O is dissolved in methanol to give the desired final ratio of TiO to RuO The solution of pertitanic acid and ruthenium trichloride are mixed and the resulting solution is applied to both sides of a cleaned titanium anode surface and to the intermediate surfaces by brushing. The coating is applied as a series of coats with baking at about 350 C. for five minutes between each coat. After a coating of the desired thickness or weight per unit of area has been applied, the deposit is given a final heat treatment at about 450 C. for fifteen minutes to one hour. The molar ratio of TiO to RuO may vary from 1:1 T102 R110 to 10:1 TiO RuO The molar values correspond to 22.3:47 weight percent Ti Ru and 51:10.8 weight percent Ti Ru.

Anodes produced according to this example will resist amalgam immersion and have a high electrochemical activity in chlorine cells which continues without material diminution over a long period of time.

The thickness of the coating may be varied according to the electrochemical needs. A typical coating to give 46 mg. Ru metal and 80 mg. titanium in the oxide coating for every 6 sq. in. of anode surface may be prepared by using 117.9 mg. RuCl .3H O (39% Ru metal) and 80 mg. of titanium metal as TiCi (80 mg. Ti dissolved in dilute HCl sufiiciently in excess to maintain acidic conditions).

EXAMPLE VI An expanded titanium anode plateof the same size as in the former examples, after cleaning and etching, was given a liquid coating containing the following materials:

Ruthenium as RuCl .H O 11.25 mg. (metal) Gold as HAuCl .nH O 3.75 mg. (metal) Titanium as TiCl 60 mg. (metal) Isopropyl alcohol 5-10 drops Hydrogen peroxide (30%) 23 drops The coating was prepared by first blending the ruthenium and gold salts in the required amount in a 2 molar solution of hydrochloric acid (5 ml.) and allowing the mixture to dry at a temperature of 50 C. The commercial solution of TiCl was then added to the Ru-Au salt mixture and a few drops of hydrogen peroxide were stirred into the solution, sufficient to make the solution turn from blue to orange. Isopropyl alcohol was finally added in the required amount. The coating mixture thus prepared was applied to both sides of the cleaned titanium anode base in eight subsequent layers, following the same heating and cooling procedure as described in Example I.

The amounts of the three metals in the coating correspond to the weight ratios of 15% Ru, 5% Au, Ti and the amount of noble metal in the coating corresponds to 0.225 mg. Ru and 0.075 mg. Au per square centimeter of projected electrode area. In accelerated testing, this anode showed a weight loss of 0.030 rug/cm. after three current reversal and a loss of 0.043 mg./cm. after two amalgam dips. After 514 hours of operation this anode showed a weight change of +0.2 m;g. /cm.

The anodes produced according to Examples I to V showed the following advantages when compared to titanium base anodes covered with platinum group metals by electroplating or chemi-deposition.

TABLE I.ACCELERATED WEIGHT LOSS TESTS Current Amalgam reversal, mgJcm.

D, Sample mg./cm. Total B (EX. II):

Ir, 0.2 mg./ern. Ru, 0.2 mgJcm. Zero Ti, 1.12 mgJcm. C (Ex. IV):

Ir, 0.2 rug/em.

Zero

Ru, 0.2 rug/cm. 0. 068 0. 068 Ti, 1.12 rug/em. D (Ex. V):

Ir, 0.2 rug/cm. Ru, 0.2 mg./em. 0.0207 0. 0138 0. 0345 Ta, 1.6 mg./em. E (Ex. VI):

Au, 0.075 mgJcm. Ru, 0.225 rug/cm. Ti, 1.2 mg./cm. RuO; coat only: On Ti base Ru, 1

mgjcm. O. 2

1 With black oxide treatment of titanium base.

Weight losses on samples prepared according to Examples I to V were determined under simulated operating conditions and compared with weight losses determined under the same conditions on titanium base samples coated with a Pt-Ir alloy. The tests were conducted in NaCl saturated solution at 65 C. and under an anodic current density of 1 A./cm. Anode potentials were measured by means of a Luggin tip against a saturated calomel electrode and converted to the normal hydrogen electrode scale. The relevant results are summarized in Table II. The integrated weight change, as shown in the next to last column, was positive, that is, increased, for most of the 9 samples prepared according to Examples I to V; which is an indication that the coating, instead of gradually wearing off and thus decreasing its precious metal oxide content, tends to build up an additional amount of protective semiconducting face which reaches stability after a short period 10 300 and 350 C. for 10 to 15 minutes, followed by fast natural cooling in air between each of the first three layers and after the fourth layer was applied the anode was heated at 450 C. for one hour under forced air circulation and then cooled.

- of Operation as shown y Sample The amounts of the three metals in the coating corre- L comragwbthe 81 sumflfilrlzed Tablefi I spond to the Weight ratios of 45% Ru, 50% Ti, 5% Ta. 3 ow 2 at even t s 6 fi er The Ta O acted as the doping agent for the TiO to iner wear m urmg Opera W sue wear crease the conductivity or semi-conductivity of the TiO rate 18 not necessarily to be imputed exclusively to the 10 in the coating spalling or washing off of noble metals, it certainly in- In acceleratd t d h d bl vol-ves also a substantial decrease of the noble metal cone 1 g 18 am e s owe no apprecla e tent in the coating. The amount of noble metals in Such weight loss after two current reversal cycles and after two noble metal alloy coatings, which is the amount necessary amalgam (1195' Each cllrrent {eve'rsal cycli conslsted of to obtain a satisfactory anode activity and a sutficiently a sefluence Of five anodlc Polarlzanons at 1 each long operating life, is from five to ten times greater than lflstlng two mmutes and followed by a cathodlc Polar in the semi-conducting rutile or tantalum oxide coatings at the Same Current n i y a d f r th same time. prepared according to the present invention. After more than 1500 hours of operation at 3 A./cm. in

i TABLE It Anode Integrated Operating potential, weight Wear rate, hours at v change, grams per Sample Coating composition 1 AJem. (NILE mg./cn1. ton Ch B (Ex. II). IrO (Ir, 0.2 mg./crn. 0 1. 62 0 RuO; (Ru, 0.2 rug/em. 792 1. 53 +0.3 0 110, (Ti, 1.12 mgJcmfl) 2, 1. 59 +0. 7 0

0 (Ex. III) Iro, (Ir, 0.4 mg./cm.*) 0 1.35 R1101 (Ru, 0.4 mgJcmJ) 860 1.36 +0. 0 0 T10, (Ti, 0.96 rug/cm!) 2, 300 1. 38 +0. 9 0

D (Ex. IV) IrO; (Ir, 0.2 mgJem!) 0 1.50 RuO (Ru, 0.2 mg./cm. 552 1. 44 +0. 75 0 110, (Ti, 1.12 mg./cm.=) 816 1.50 +0. 4 0

E (Ex. V) IrOi (Ir, 0.2 rug/0111!).-- 0 1.45 R1101 (Ru, 0.2 mg./cm. 514 1.45 -0. 007 0.15 T302 (Ta, 1.6 mgJcmJ) F (Ex. VI) AuzO3(AI1, 0.075 mgJcmfi) 0 1. 48 R110, (Ru, 0.22.5 m ./cm.=) 514 1. 48 +0. 2 0 T102 ('Ii 1.2 rug/cm?) G Pt (1.44 rug/cm!) 0 1.36 Ir (3.36 mg./cm.=) 1, 032 1. 4s =-0. 0. 26 2, 370 1. 5s -o. 9 0. 32

H Pt (3.68 mgJcm!) 0 1. 39

Ir (0.92 rug/c1113)-.- 926 1. 35 2, 940 1. 39 0. 6 0. 1s

1 Weight increase. 1 Increase. 5 Decrease.

The average thickness of the final coating of Examples I to V is 1.45 microns or 57 micro-inches and the ratio of platinum group metals to non-precious metals in the oxide coatings of the catalytically active semi-conductor coatings of Examples I to V may be between 20 to 100 a and 85 to 100.

EXAMPLE VII An expanded titanium anode plate was submitted to a cleaning and etching procedure and then given a liquid coating containing the following materials:

Ruthenium as RuCl 3H O 0.8 mg./cm. (metal) Titanium as TiCl 0.89 mg./cm. (metal) Tantalum as TaCl 0.089 mg./cm. (metal) The coating mixture was prepared by first blending the dry ruthenium salt in the commercial hydrochloric acid solution containing 15% TiCl Tantalum was then added in the above proportion and in the form of a solution of g./l. TaCl in 20 %HCl. The blue color of the solution was made to turn from blue to orange by introducing the necessary amount of hydrogen peroxide, which was followed by an addition of isopropyl alcohol as a thickening agent. The coating mixture was applied to both sides of the titanium anode base by electrostatic spray coating in four subsequent layers. The number of layers can be varied and it is sometimes preferable to apply several coats on the area facing the cathode and only one coat, prefer ably, the first coat, on the area away from the cathode. After applying each layer, the anode was heated in an oven under forced air circulation at a temperature between saturated sodium chloride solution, the anode potential was 1.41 V.

EXAMPLE VIII An expanded titanium anode plate was submitted to a cleaning and etching procedure and then given a liquid coating containing the following materials:

Ruthenium as RuCl .3H O 0.6 mg./cm. (metal) Titanium as TiCl 0.94 mg./cm. (metal) Tin as SnCl 0.17 mg./cm. (metal) The coating was prepared by first blending the dry ruthenium salt in the commercial hydrochloric acid solution with 15% TiCl Tin tetrachloride was then stirred into the mixture in the above proportion, followed by sufiicient hydrogen peroxide to cause the blue color of the solution to turn to orange. Isopropyl alcohol was added as a thickening agent. The coating mixture was applied to both sides of the pre-cleaned and pre-etched titanium anode base in four subsequent layers and each layer was submitted to the usual thermal treatment as described in Example VII. The amounts of the three metals in the coating correspond to the weight ratios of 35% Ru, Ti, 10% Sn. In accelerated testing the anode showed a weight loss of 0.09 mg./cm. after two current reversal cycles as described in Example VII and a weight loss of 0.01 mg./cm. after one amalgam dip. After more than 1500 hours of operation in concentrated NaCl solution at 2 A./cm. and C., the anode potential was 1.42 V.

1 1 EXAMPLE IX A pre-cleaned titanium anode plate was coated with a coating mixture consisting of a hydrochloric acid solution containing the following salts:

Ruthenium as RuCl .H O 0.8 mg./cm. (metal) Titanium as TiCl 0.96 mg./cm. (metal) Aluminum as AlCl .6H O 0.018 mg./cm. (metal) The mixture was prepared by first blending the ruthenium and titanium salts in the commercial hydrochloric acid solution of TiCl as described in the former examples. Aluminum trichloride was added in the above proportion, followed by treatment with hydrogen peroxide as in Example VII and isopropyl alcohol was added as a thickening agent. The mixture was applied to the precleaned and pre-etched titanium anode base in four subsequent layers, taking care to apply the coating to both sides of the base and to the exposed areas between the top and bottom surfaces of the anode base. Thermal treatment procedure after each layer was as described in Example VI.

The amounts of the three metals in the coating correspond to weight ratios of 45% Ru, 54% Ti and 1% Al. After one current reversal cycle and two amalgam dips, the overall weight loss was 0.1 mg./cm After operating for more than 1500 hours in concentrated sodium chloride solution at 60 C. under an anodic current density of 3A./cm the anode potential was 1.42 V.

X-ray diffraction analysis indicates that the coatings on the above anodes are in the form of semi-conducting rutile in which the doping oxides have become diffused in the rutile crystals by solid solution to give the valve metal anode base a semi-conducting rutile face with a chlorine discharge catalyst with ability to oxidize dissolved chloride lions to molecular chlorine gas. The chlorine discharge catalyst is preferably an oxide of a platinum group metal. The coatings may be applied and fixed upon tantalum electrode bases in a similar manner.

While semi-conducting faces may be applied to titanium or tantalum bases with other doping compositions, our tests so far have shown that when using the formulations and deposition methods described, the presence of titanium or tantalum oxide and iridium alone, i.e., without ruthenium oxide, give a deposit of low electrocatalytic activity with a higher chlorine discharge potential.

EXAMPLE X The coating mixture consisted of an HCl solution containing the following salts:

Manganese as Mn(NO )2 0.5 mg./cm. (metal) Tin as SnCl .5H O 0.5 mg./cm. (metal) The solution was prepared by first blending the two salts in 0.5 ml. of 20% HCl of 20% HCl for each mg. of overall salt amount, and then adding 0.5 ml. of formamide. The solution was seated at 40-45 C. until reaching complete dissolution, and then applied in six subsequent coatings on the pre-etched titanium base with a thermal treatment after each layer as formerly described. The anodic potential under chlorine discharge in saturated brine at 60 C. was 1.98 v. at the current density of 1 A./cm.

EXAMPLE XI Using the same procedure as described in Example IX, the following binary salt mixture was applied to the titanium base electrode:

Molybdenum as Mo (NH O 0.5 mg./cm. (metal) Iron as FeCl 0.5 nag/cm? (metal) The anodic potential measured as in Example IX was 2. O.V.

12 EXAMPLE XII Using the same procedure as in Example IX, the following binary mixture was applied to a titanium base electrode:

Cobalt as CoCl 0.5 mg./cm. (metal) Antimony as SbCl (COOH) (CHOI-l) 0.5 mg./cm. (metal) The anodic potential measured as in the former examples was 2.05 v.

EXAMPLE XIII The binary mixture applied to the titanium base electrode according to the protcedure of former Example IX was as follows:

Rhenium as (NHQ ReCI 0.5 mg./cm. (metal) Iron as FeCl 0.5 mg./cm. (metal) The anodic potential measured as in the former examples was 1.46 v.

EXAMPLE XIV The binary mixture applied to the titanium base electrode consisted of the following salts:

Rhenium as (NH ReCl 0.5 mg./cm. (metal) Manganese as Mn(NO 0.5 mg./cm. (metal) The mixture was prepared and applied following the same procedure as described for the former examples, with multiple coats with heating between each coat and after the final coat. The anodic potential in saturated sodium chloride brine at 60 C. and at 1 A./cm. was 1.8 v.

EXAMPLE XV The binary mixture applied to the titanium base electrode consisted of the following salts:

Rhenium as (NHQ ReCl 0.5 mg./cm. (metal) Zmc as ZnCl 0.5 mg./cm. (metal) It was prepared and applied as described in Example IX. The anodic potential under the same conditions was 1.40 v.

EXAMPLE XVI A mixture of three salts in HCl solution was applied to the titanium base electrode, as follows:

Rhenium as (NH ReCl 0.4 mg./cm. (metal) Iron as FeCl 0.3 mg./cm. (metal) Tin as SnCl 5H O 0.3 mg./cm. (metal) Electrodes were made with five different coating types, each of which consisted of a four-component salt mixture including a ruthenium salt.

Sample No. 1

Weight percent metal 1.14 mgJcm. (metal) Ti 0.071 mgJcm. (metal) 4% V Tantalum as 'IaCls in 11C]. 0.017 mgJcm. (metal). 1% Ta solution (commercial). Ruthenium as RuCl;-31IzO. 0.53 mgJem. (metal) 30% Ru Titanium as TiCl; iu H01 solution (commercial). Vanadium as VOClz-2H O in HCl solution (commercial).

13 14 Sample No. 2 Sample No. 2

Wright Weight percent percent metal metal Titainitum 22s TiCl; in E01 1.06 mgJcm. (metal) 60% Ti Tisllofliltllltl lnrlx a(s TiCl; in gCl 0.7 mgJcm. (metal) 39.2% Ti 8011 Ion commerela 1O commence Tanltalum Ezs T8015 iuDHCl 0.088 mgJcm. (metal)..... 5% Ta Lzglghgnum as La(NO;);- 0.088 mgJem. (metal) 4.9% La S llt' I I'- 'Iiri as Sll Clf5 llTO i i. do Sn Tin ZS SnCh-SHgO 0.15 rug/em. (metal) 8.4% Sn Ruthenium as RuCh3H 0.53 rug/em. (metal) 30% Ru Rhodium as (N H4); RhClt 0.85 mgJem. (metal) 47.5% Rh Sample No. 3 Sample No. 3

Weight Weight percent percent metal metal Titanium as TiCh in H01 0.96 mgJcm. (metal) 53.0% Ti Titanium as Tioll in HCl 0.7 mgJcm! (metal) 39.2% Ti solution (commercial) solution (commercial). Lanthanumas La(NOs):-3HzO 0.071 rug/em. (metal) 3.0% a Aluminum as AlCl3-6H2O 0.088 mgJern. (metal) Tin as Sl'lCl -5Hz0 0.25 lug-[c111- (metal) 3% S11 Till as SnC1l-5H2O 0.15 mgJem. (metal) Ruthenium as Ruck-SE10..-" 0.53 mgJem. (metal) 9 R11 Iridium as IrCl; 0.85 mgJem. (metal).

Sample No. 4 Sample 4 Weight W ht ptfr ent fs 3 metal 2 Titainitum 12.5 'IiCla in 501 0.7 mgJem. (metal) 39.2% Ti 11 0 m Tlstagifi egg-gist? 1.07 mgJemJ (metal) 6 Ti gfg gf ii gfia i:( %;l )1 gig, 811 ll]. 35 ll m 0111. me 1 l1 ,;%,i 6fi?5t?ffff2::f1fl fitfffiffi ff fjjjjj ZZZ, 5 Palladium a; w lug/cw Pd Ruthenium as Ruck-311 0"-.- 0.53 mgJem. (metal Ru The four mixtures were applied on five separate tita- 3 1 N 5 30 nium and on five separate tantalum plates in five subseamp quent coatlngs. Intermedlate and final heat treatments were given as in Example XVII. The anodic tentials, Weight p0 percent measured under the same conditions as in the former exmetal ample, were the following: Titalnluln a(s TiCh in (5)51 0.88 nag/em. (metal) 78.0% Ti S 1 N 1 1 so ution commercia amp 6 t l 7.87 Al l%i -5 9 f E 9- 0 088 Egg lm (me 8) 7 8%? Sn Sample No. 2 1,85 Ruthenium as RuCh. 3H10. 0.071 nag/cm. (metal 11 Sample No 3 137 40 Sample No. 4 1.39 Each sample was prepared by first blendillg the l EXAMPLE XIX um salt 1n the commercial hydrochlorlc acld solutlon of TiCl and adding hydrogen peroxide in the amount re- Electrode? were "f Wlth five dlfiefent Coatlng fYP quired to obtain a color change from blue to red. To this F Q whlch colfslsted of a f "Q P $81! mixture mixture were added the other salts in the stated propor- Including a futhemllm 0 a ltanlum salt and a salt of tions plus 056 m1. of isopropanol f each f overall a metal having an atomic valence dliferent from tltamum metal amount. The five mixtures were applied on five sepand flctmg as a pl g g nt f r titanium dlox de. These arate titanium plates in five subsequent coatings. Heat coafmgs were PP to l expanded ltamllm sheet treatment at 350 C. for 10 minutes was given between Whwh had f cleaned y bplllng at a reflux P f each coating and the next A final treatment at 5 of 109 C. m a 20% solution of hydrochloric acid for for 1 hour followed the last coating 5 20 mlnutes, 1n the amounts specified per square centi- Anodic tests were carried out in saturated NaCl brine meter of Prolected electrode areaat 60 C. at a current density of 1 A./cm. The measured Sample 1 electrode potentials were as follows:

v Weightt percen Sample No. 1 1.42 metal samplfl 2 L40 Rutheniumas Bums-31110---. 1.60 mgJcmJ (metal) 45% Ru Sample No. 3 1.39 AluminumasAlCla-fiHzo- 0.036 mgJcmJ (metal)- 1% Al 1 44 Tin as SnCh-5I zO- 0.142 mglem. (metal).- 4% Sn Sample 4 Titanium as TlCl3 1.78 mgJcm. (metal) 50% Ti Sample No. 5 1.39

EXAMPLE XVIII This coating was prepared by first blending ruthenium, Four coating types were tested, each of which congg f gg z gg g l g gggffg figfigg fik sisted of a four component salt mixture, including a noble 1 0 t 81 Salt s owly added under stlrrlng. me Sam 16 No 1 After the salts were completely dissolved, a few drops P of hydrogen peroxide (H 0 30%) were added, sufficient to make the solution turn from the blue of the commers cial TiCl solution to the brown-reddish color of a peroxyhydrate compound.

At the end, a few drops of isopropyl alcohol are added iioil rtiaif (metal) 392% T1 to the solution after cooling. The coating, thus prepared, Lant anumasLa( 0s)r g-/ L8 was applied to the working side of the cleaned titanium ffg Q15 mg/cml .4 s expanded mesh surface by brush or spraying in 10 to 14 Platin m as PtCh- Hz0 -8 s-l tal) 47. Pt subsequent layers. After applying each layer, the sample (commercial).

was heated in an oven under forced air circulation at a temperature between 300 to 400 C. for to minutes, followed by fast natural cooling in air between each of the first 10 to 14 layers and after the last coat was applied, the sample was heated at 450 C. for one hour under forced air circulation and then cooled.

In our standard accelerated testing, this sample showed a weight loss of Zero after current reversals and a loss of 0.2 to 0.3 mg.%cm. after three amalgam dips. After 11,000 hours of operation at 30 kA./rn. in saturated NaCl brine and 60 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.38 v. (NHE).

Sample No. 2

This coating was applied to a cleaned titanium expanded mesh anode base according to the procedure of Sample No. l, and consisted of the following materials:

Tin as SnCl4-5H1O 0.142 mgJern. (metal) 4% Sn Titanium as TiCl; 1.78 nigJcm. (metal) 50% T1 Solution (commercial).

The procedure for compounding the coating and applying it to the Ti base was the same as in Sample No. 1.

In our standard accelerated test, this sample showed a weight loss of zero after current reversals and a loss of 0.2 to 0.4 mg./cm. after three amalgam dips. After 10,000 hours of operation as an anode at 30 kA./m. in saturated NaCl brine and 65 C., the electrode showed a weight loss of zero and an anode potential of 1.36 to 1.37 v. (NI-IE).

This coating has shown a higher electrocatalytic activity than any other formulation not containing iridium. A portion of the mixture Co O +CoO may reverse the electrical conductivity of the semi-conductor from n to p type and other portions of the Co O +CoO mixture may produce a spinel with Sn0 introducing new electrocatalytic sites into the coating.

Sample No. 3

The coating was applied to a cleaned Ti anode base according to the procedure of Sample No. 1, and consisted of the following amounts:

Weight percent metal Ruthenium as Bums-31110- 1.6 mgJcm. (metal) 45% Ru Lanthanurn as LaCh-9HzO. 0.036 mgJem. (metal) 1% La Tin as SnC14-5HO 0.142 ing/em. (metal). 4% Sn Titanium as T101; 1.78 rug/em. (metal) 50% T1 Sample No. 4

This coating mixture was applied to a cleaned Ti anode base according to the procedure of Sample No. 1 and consisted of the following materials:

Weight percent me tal Ruthenium as RuCl 3H O 1.60 rngjem. (metal) 45% Ru Vanadium as VaClz solution 0.036 mg./cm. (mctal)..... 1% Va (commercial).

Tantalum as TaChH 0.142 rug/cm. (metal). 4% Ta HCl solution, 0.02 mg. Ta/u.).

Titanium as 15% TiCl;

solution (commercial).

1.78 ing/cm? (metal) 50% T1 This coating was applied to a cleaned Ti base according to the procedure of Sample No. 1 and consisted of the following materials:

Weight percent metal Ruthenium. as RUCl3-3Hz0 1.6 mg./em. (metal) Ru Chromium as Cl(N a)a-8Hz0- 0.036 mg/em. (metal) 1% Cr Tin as SnCl1-5HgO 0.142 ing/cm. (metal). 4% Sn Titanium as 15% TiCh solu- 1.78 mgJcm. (metal) Ti tion (commercial).

The procedure for compounding the coating and applying it to the Ti base was the same as in Sample No. 1. After 5,000 hours of operation as an anode, the sample showed a weight loss of Zero and an anode potential of 1.37 v. (NHE) at 30 mA./m. in saturated NaCl brine and C.

Each sample was prepared by blending the three salts first enumerated under each of Samples 1 to 5, in the required amounts. The titanium chloride solution (15% as TiCl in commercial solution) was then slowly added to the blended mixture of the first three salts under stirring. After all the salts were completely dissolved, a few drops (3 to 5) of hydrogen peroxide (H 0 50%) were added, sufficient to make the solution turn from the blue of the commercial TiCl solution to the brown-reddish color of a peroxyhydrate compound. At the end of the mixing, a few drops of isopropyl alcohol were added to the solution after cooling.

The coatings thus prepared were applied to the working side of the cleaned titanium surface by brush or spraying 10 to 14 subsequent layers. After each layer, the sample was heated in an oven under forced air circulation at a temperature between 300 to 400 C. for 5 to 10 minutes, followed by fast natural cooling in air between each layer and after the last layer was applied, the sample was heated at 450 C. for one hour under forced air circulation and then cooled.

In standard accelerated tests the samples showed the following weight loss.

Weight loss after current reversals Weight loss after 3 amalgam dips Sample number:

1 0 0.2 to 0.3 ing/cm I O 0.2 to 0.4 mg./cm t 0 0.3 to 0.5 mg./cm 2 0 0.35 to 0.5 mgJcm 7 0 0.25 to 0 4 mg./cm 3 Hours of Weight operation loss Anodic potential 11,000 0 1.38 v. 10, 000 0 1.36 to 1.37 v. (NHE). ,000 0 1.39 to 1.40 v. (NI-IE). 11,000 0 1.38 v. (NHE). 5.000 0 1.37 v (NHE) EXAMPLE XX An expanded titanium sheet was etched with boiling HCl 20% solution at reflux temperature (109 C.) for 40 Hydrogen peroxide, H20 30%. 3 to 5 drops lsopropyl alcohol 4 to 6 drops CI13C110I'ICII3- 99%.

This coating was prepared by blending the ruthenium, iron and tin salts in the required amounts and the TiCl solution (15% TiCl in commercial solution) was slowly added under stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 30%) were added, sufiicient to make the solution turn from the blue color of commercial TiCl solution to the brownreddish color of a peroxyhydrate compound. After cooling, a few drops of iospropyl alcohol were added.

The coating, thus prepared, was applied to the working side of the etched titanium surface by brush or spraying in to 14 subsequent layers. After applying each layer, the sample was heated in an oven at a temperature of 300 to 400 C. for 10 minutes, followed by fast natural cooling in air between each of the first 10 to 14 layers.

After the last layer was applied, the sample was heated at 450 C. for 1 hour under forced air circulation and then cooled.

- On standard accelerated testing, the sample showed a Weight loss of 0.2 mg./cm. after three amalgam dips. After 10,000 hours of operation at 30 kA/m. in saturated brine at 65 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.40 v. (NHE).

Sample No. 2

An expanded titanium sheet was etched as described above, and was then coated with the following mixture:

Wei ht percent metal This coating was prepared by first blending the ruthenium, nickel, cobalt, chromium and tin salts in the required amounts and the TiCl solution was slowly added under stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 0 30%) were added, suflicient to make the solution turn from the blue color of commercial TiCl solution to the brown-reddish color of a peroxyhydrate compound. After cooling, a few drops of isopropyl alcohol were added.

The coating, thus prepared, was applied to the working side of the etched titanium according to the procedure used for the preceding Sample No. 1.

On accelerated tests, the sample showed a weight loss of 0.25 to 0.3 mg./cm. after three amalgam dips and a weight loss of zero after current reversals.

After 5,000 hours of operation at 30 kA./m. in saturated brine at 65 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.37 v. (NHE).

Sample No. 3

An expanded titanium sheet was etched as described for sample No. 1, and was then coated with the following mixture:

W91 ht percent metal This coating was prepared by first blending the ruthenium, nickel, iron, cobalt and chromium salts in the required amounts and then adding the TiCl solution slowly with stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 0 30%) were added, sufficient to make the solution turn from the blue color of commercial TiCl solution to the brownreddish color of a peroxyhydrate compound. After cooling, a few drops of isopropyl alcohol were added.

The coating, thus prepared, was applied to the working side of the etched titanium according to the procedure used for the preceding sample No. 1.

On accelerated tests, the sample showed a weight loss of zero after current reversals and a loss of 0.2 to 0.3 mg./cm. after three amalgam dips.

After 5,000 hours of operation at 30 kA./m. in saturated brineat 65 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.38 v. (NHE).

EXAMPLE XXI An expanded titanium sheet was etched with boiling HCl 20% solution at reflux temperature (109 C.) for 40 minutes and then coated with the following:

Sample No. 1

Weight percent metal Ruthenium as RuCh-3H' O 1.6 mgJem. (metal)..- 45% Ru Nickel as NiClz-GHzO 0.178 mgJcm. (metal). 5% Ni Titanium as TiCla 1.78 mgJcrn. (metal). 50% T1 Hydrogen peroxide, H 0 30%- 3 to 5 Isopropyl alcohol 4 to 6 CH3CHOHCH1. 99%.

This coating was prepared by blending the ruthenium, nickel and titanium salts in the required amounts and the TiCl solution (15% TiCl in commercial solution) was slowly added under stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 0 30%) were added, suflicient to make the solution turn from the blue color of commercial TiCl; solution to the brown-reddish color of a peroxyhydrate compound. After cooling, a few drops of isopropyl alcohol were added.

The coating thus prepared was applied to the working side of the etched titanium surface by brush or spraying in 10 to 14 subsequent layers. After applying each layer, the sample was heated in an oven at a temperature of 300 to 400 C. for 10 minutes, followed by fast natural cooling in air between each of the first 10 to 14 layers.

After the last layer was applied, the sample was heated at 450 C. for 1 hour under forced air circulation and then cooled.

On standard accelerated testing, the sample showed a weight loss of zero after three amalgam dips. After 5,000 hours of operation at 30 kA./m. in saturated brine at 65 C. the electrode as an anode showed a weight loss of zero and an anode potential of 1.38 v. (NI-IE).

Sample No. 2

An expanded titanium sheet was etched as described for sample No. 1, and was then coated with the following mixture:

Weight percent metal Ruthenium as RuClg-3H;O- 1.6 mgJem. (metal) 45% Ru Cobalt as COClz-GHzO 0.178 mgJcm. (metal) 5% Co Titanium as 'IiClz 1 7R mgJcm. (metal) 50% Ti Hydrogen peroxide, H201, 30%- 3 to 5 drops Isonropyl alcohol 4 to 6 drops CHaCHOHCHa, 99%.

This coating was prepared by first blending the rutheniurn and cobalt salts in the required amounts and the TiCl solution was slowly added under stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 0 30%) were added, sufiicient to make the solution turn from the blue color to commercial TiCl solution to the brown-reddish color of a peroxyhydrate 19 compound. After cooling, a few drops of isopropyl alcohol were added.

The coating, thus prepared, was applied to the working side of the etched titanium according to the procedure used for the preceding Sample No. 1.

After accelerated tests, the sample showed a weight loss of zero after current reversals and a loss of 0.2 mg./cm. after three amalgam dips.

After 5,000 hours of operation at 30 kA./m. in saturated brine at 65 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.38 v. (NHE).

Sample No. 3

An expanded titanium sheet was etched as described for sample No. 1, and was then coated with the following mixture:

This coating was prepared by first blending the ruthenium and iron salts in the required amounts and then adding the TiCl solution slowly with stirring. After the salts were completely dissolved, a few drops of hydrogen peroxide (H 30%) were added, suflicient to make the solution turn from the blue color of commerical TiCl solution to the brown-reddish color of a peroxyhydrate compound. After cooling, a few drops of isopropyl alcohol were added.

The coating, thus prepared, was applied to the working side of the etched titanium according to the procedure used for the preceding sample No. 1.

After accelerated tests, the sample showed a weight loss of zero after current reversals and a loss of 0.2 to 0.3 ing/cm? after three amalgam dips.

After 5,000 hours of operation at 30 kA./m. in saturated brine at 65 C., the electrode as an anode showed a weight loss of zero and an anode potential of 1.38 v. (NHE).

While we have given some theories to better describe our invention, these are for explanation only and we do not intend to be bound by these theories in the event it is shown that our invention works differently from the theories given.

The word oxide in the following claims is intended to cover oxides of titanium and tantalum whether in the form of TiO and Ta O or other oxides of these metals and oxides of other metals capable of forming semi-conduc tive coatings with oxides of metals from adjacent groups of the Periodic Table, and the words noble metals is intended to include the platinum group metals and gold and silver. The titanium dioxide may be in rutile or anatase form.

The base of the electrode may be a valve metal or any metal capable of withstanding the corrosive conditions of an electrolytic chlorine cell, such as high silicon iron (Duriron), cast or pressed magnetite, etc. Our preference, however, is for a titanium or tantalum base.

The electrodes of our invention may be used in any liquid phase or gaseous phase electrolyte, particularly aqueous salt solutions or fused salts. They are dimensionally stable and are not consumed in the electrolytic process and when used in alkali halide electrolytes such as, for example, sodium chloride solutions used for the production of chlorine and sodium hydroxide, our electrodes form the anodes and the cathodes may be mercury, steel or other suitable conductive material. In mercury cells such as typified, for example, in US. Pat. No. 3,042,602 or No. 2,958,635, or in diaphragm cells such as US. Pat. No, 2,987,463, our electrodes are the anodes and are used in place of the graphite anodes shown in these patents and heretofore used in cells of this type.

The semi-conductor coatings conduct the electrolyzing current from the anode bases to the electrolyte through which it fiows to the cathode.

Various modifications and changes may be made in the steps described and the solutions and compositions used without departing from the spirit of our invention or the scope of the following claims.

What is claimed is:

1. The method of producing an electrode of a valve metal base from the group consisting of titanium and tantalum which comprises applying a coating mixture in liquid form to said valve metal base, which on heating forms an oxide layer on said base, 39.2% to 78% of which comprises an oxide of titanium, 6.4% to 47.5% of which comprises an oxide of a platinum group metal and 1% to 17.7% of which forms an oxide of a doping metal from the group consisting of tin, vanadium, lanthanum, cobalt, and mixtures thereof, the said percentages being based upon the weight of the metals in said oxides, applying said coating in several separate layers and heating the coating on the valve metal base between the application of each layer.

2. The method of producing an electrode of a valve metal base from the group consisting of titanium and tarltalum which comprises applying a coating mixture in liquid form to said valve metal base, which on heating forms an oxide layer on said base, 39.2% to 78% of which comprises an oxide of titanium, 6.4% to 47.5% of which comprises at least one oxide of a platinum group metal oxides and the remainder of which comprises an oxide of tin and the oxide of one or more metals from the group consisting of tantalum, lanthanum, chromium, aluminum, iron, cobalt and nickel, the said percentages being based on the weight of the metals in said coating, applying said coating in several separate layers and heating the coating on the valve metal base between the apptication of each layer and after application of the final lever.

It. The method of claim 2, in which the heating between the layers is at about 300 to 350 C. for about 15 minutes and after the final layer at about 450 C. for about one hour.

4. The method of claim 2, in which said remainder oi mprises an oxide of tin in amounts of 1 to 13.8% and an oxide from the group consisting of cobalt, nickel, iron, tantalum and mixtures thereof in an amount of 1 to 5%.

5. The method of claim 4, in which the coating mixture contains 50% to 65% of titanium, 30% to 45% of ruthenium, and approximately 1% to 10% of metal from the group consisting of tin and cobalt, said percentages being based upon the weight of the metals in said coating and the metals in said coating being'in the form of oxides.

6. The method of claim 4, in which the coating mixture contains approximately 50% of titanium, approximately 45 of ruthenium and approximately 5% of tin and cobalt, said percentages being based upon the weight of the metals in said coating.

7. The method of claim 2, in which said coating includes two or more platinum group metal oxides.

8. The method of claim 2, in which said remainer consists of 1% to 13.8% of tin oxide and 1% to 5% of an oxide from the group consisting of cobalt, nickel, iron, tantalum, aluminum, chromium, and mixtures .thereof.

9. The method of claim 2, in which said remainder comprises 1% to 5% of an oxide of cobalt and of an oxide from the group consisting of tin, chromium, iron, nickel and mixtures thereof.

10. A process for making electrodes which comprises preparing a solution containing a solvent from which at least three metal oxides can be deposited by precipitation and heating, said oxides comprising 39% to 78% of an oxide of titanium, 16% to 47.5% of oxides of platinum group metals and 4% to 17.7% of an oxide of tin, vanac li 21 um, cobalt, and mixtures thereof, said percentages being based upon the weight of the metals in said oxides, applying the solution to a conductive electrode base, heating the base to drive off the solvent, to deposit the metal oxides on the base and convert them into a semi-conductor coating on the base.

11. The method of claim 10, in which the solution is deposited in multiple layers on the base and heated between each layer application and after depositing the final layer.

12. The method of claim 11, in which the heating between application of the layers is at about 350 C. and after the final layer at about 450 C.

13. The method of claim 10, in which the said 4% to 17.7% amount is of tin and one or more non-precious metals from the group consisting of cobalt, nickel and Iron.

14. The method of producing an electrode comprising a chlorine resistant metal base having a semi-conductor coating thereon containing (a) a platinum group metal oxide, (b) titanium dioxide and (c) a doping oxide for said titanium dioxide from the group consisting of oxides of tin, lanthanum, aluminum, cobalt, antimony, molybdenum, tungsten, tantalum, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, and mixtures thereof, the titanium dioxide in said coating constituting more than 50% of the total metals in said coating, the platinum group metal oxide constituting from 16% to 47.5% of the total metals in said coating and the doping oxide constituting from 4% to 17.7% of the total mr -ta's in said coating, which comprises applying a coating mixture in solution form to said metal base, said coating mixture containing a titanium compound in the form of pertitanate, a platinum group metal compound and doping metal compounds in solution form which on heating form oxides of the metals in said compounds and form a semiconductor of the said titanium compound and said doping metal compounds, applying said coating mixture in several separate layers and heating the coating on said metal base between the application of each layer and after application of the final layer.

15. The method of claim 14, in which the chlorine resistant metal base is titanium, the platinum group metal compound is a ruthenium compound and the doping metal compound is from the group consisting of cobalt, tin, nickel, aluminum and lanthanum, and mixtures thereof.

References Cited UNITED STATES PATENTS 3,562,008 2/ 1971 Martinsons 117221 FOREIGN PATENTS 1,195,871 6/ 1970 Great Britain 204290 F 6606302 11/1966 Netherlands 204290 F TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R.

3,846,273 November 5, 1974 Patent No. Dated Giuseppe Bianchi et a1. lnventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 46, "TaO" should read ra o Column 4, line 52, "NiO" should read NiO Column 6, lines 48 to 51 (second column of table listing metals) 48 mg. (metal) 4 20 mg. (metal) 10 to 12 drops '48 mg. (metal) 3 to 4 drops Should read, 10 to 12 drops 20 mg. (metal) 3 to 4 drops line 72 "Wa sused" should read was used Column 11, line 36, 'lions" should read:- ions line 56, "of 20% HCl", second occurrence, should be deleted; line 58, "seated" should read heated Column 13, line 37, "/m should read /cm line 39, Ku" should read Ru Column 16, line 38, before "10" insert "a in Column 6, lines 44 and 54, "II", each occurrence, should read I Signed and sealed this 4th day of February 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Arresting Officer Commissioner of Patents FORM o-wso (10- UcoMMDC wan'peg U.S, GOVERNMENT PRINTING OFFICE; 

1. THE METHOD OF PRODUCING AN ELECTRODE OF A VALVE METAL BASE FROM THE GROUP CONSISTING OF TITANIUM AND TANTALUM WHICH COMPRISES APPLYING A COATING MIXTURE IN LIQUID FORM TO SAID VALVE METAL BASE, WHICH ON HEATING FORMS AN OXIDE LAYER ON SAID BASE, 39.2% TO 78% OF WHICH COMPRISES AN OXIDE OF TITANIUM, 6.4% TO 47.45% OF WHICH COMPRISES AN OXIDE OF A PLATINUM GROUP METAL AND 1% TO 17.7% OF WHICH FORMS AN OXIDE OF A DOPING METAL FROM THE GROUP CONSISTING OF TIN, VANADIUM, LANTHANUM, COBALT, AND MIXTURES THEREOF, THE SAID PERCENTAGES BEING BASED UPON THE WEIGHT OF THE METALS IN SAID OXIDES, APPLYING SAID COATING IN SEVERAL SEPARATE LAYERS AND HEATING THE COATING ON THE VALVE METAL BASE BETWEEN THE APPLICATION OF EACH LAYER. 